This invention relates to a method and apparatus for automatically optically inspecting a glass web, and more particularly to such a method and apparatus which utilizes internal reflection moving from defects in the web toward the edges and edge detecting the totally internally reflected light at the edges thereof.
The imposition of stringent quality restrictions on the manufacture of float glass presents a serious inspection problem. The type of flaws, the number and where they are located within the glass web is a difficult task for human inspection. With insufficient product knowledge, it is difficult to determine what portion of the glass must be scrapped, which can be salvaged and how to prevent wasted time and materials in the manufacturing process.
Amont the many problems are the detection of bubbles in the glass which are caused by gases captured by the molten elements during the heating process. Such bubbles do not have time to emerge before the glass leaves the heating process. The bubbles vary in size from microns to several centimeters and their size and number vary with the thickness of the glass. Bubbles may be classified as filled bubbles or salt cakes which look like stones and occur infrequently; body bubbles which are small in diameter, relative to the thickness of the glass, which do not distort the glass and appear as small, spherical or eliptical lines and block transmitted light in the glass; or surface bubbles which tend to appear just below the surface of the glass, mostly on the lower surface in float glass and cause distortion in the glass and as an extreme case provide an open bubble when the bubble breaks on the surface.
Bubbles vary greatly in shape from spherical to ellipsoidal with the larger bubbles tending to be more elongated as well as those which are generated in higher speed glass lines. The most important factors in determining the bubble shape are the operating characteristics of a particular float line for manufacturing the glass.
Traditionally bubble detection is done visibly by inspectors with the glass being lit or illuminated from the top such that the defect casts a characteristic shadow on a white board under the glass web. Through training and experience glass inspectors can learn to identify certain kinds of flaws. When a flaw is identified, the location must be marked with a colored crayon attached to the end of a long pointer. However, a great deal of subjective judgment is involved, the accuracy depends on the experience of the inspector, the speed of the line and of course the foibles of human perceptions and response.